Catalytic processes that are used for the conversion of hydrocarbonaceous streams into a hydrogen-rich gas, e.g. steam reforming, autothermal-reforming, and catalytic partial oxidation, are extremely sensitive to sulphur. Desulphurised hydrocarbonaceous streams that are obtained after conventional hydrodesulphurisation processes using Co—Mo or Ni—Mo catalysts generally have a too high sulphur level to be suitable for these sulphur-sensitive applications. Hydrodesulphurised hydrocarbonaceous streams contain sulphur compounds that are difficult to remove, such as heterocyclic sulphur compounds like thiophenes, benzothiophenes, substituted and condensed ring dibenzothiophenes. Therefore, there is a need for catalysts that are able to remove the small amount of “difficult” sulphur compounds that are still present in hydrocarbonaceous streams that have undergone a conventional hydrodesulphurisation.
Catalysts containing nickel, zinc oxide and alumina have been reported for deep desulphurisation of hydrocarbon streams, i.e. to sulphur concentrations as low as 0.1 ppm. Generally these catalysts have a double function: nickel catalysts the reaction of sulphur with hydrogen to form hydrogen sulphide and zinc oxide absorbs the hydrogen sulphide formed by reaction to zinc sulphide and water. Alumina is needed for the strength of the catalyst particles and it creates a relatively high specific surface area which is needed for a good nickel dispersion. These catalysts are able to remove heterocyclic sulphur compounds.
In EP 1 224 970 for example, such a double function catalyst for the deep desulphurisation of kerosene is described. The catalyst has 5-25 wt % Ni, 30-70 wt % ZnO and the remainder alumina. The catalyst is prepared by precipitating a water-soluble nickel salt and a water-soluble zinc salt in a basic solution, admixing alumina or an alumina precursor with the precipitate and calcining the admixture.
In Ind. Eng. Chem. Res. 40 (2001) p. 2367-2370, a similar double function catalyst comprising nickel, zinc oxide and alumina is described. The Zn:Ni ratio of the described catalysts is below 12.
In U.S. Pat. No. 6,254,766, a sorbent composition for the removal of “difficult” sulphur compounds is disclosed. The sorbent composition comprises zinc oxide, silica, alumina and nickel in a substantially reduced valence state. The sorbent preferably contains 15-60 weight % zinc oxide and 15-40 weight % nickel. The exemplified catalyst has a Zn:Ni ratio far below 12. A process for the removal of organosulphur from a stream of cracked-gasoline or a diesel fuel using this sorbent composition is described. After contacting the stream with the sorbent composition, the sorbent composition is regenerated by reducing it in an activation zone.